In order to evaluate engine performance, the ignition signals or pulses which sequentially fire the spark plugs of an internal combustion engine are evaluated. A number of malfunctions relating to ignition can be uncovered by viewing the ignition signals on an oscilloscope. In currently available oscilloscopes, the horizontal sweep is triggered simultaneously with the commencement of the ignition signal so that the ignition signal appearing on the oscilloscope screen commences at the left-hand side of the screen. There is or may be useful information between the end of one sweep and the commencement of the next sweep, which information is lost or at least not readily apparent on the screen. It has been found to be desirable to advance commencement of the horizontal sweep so that the start of the ignition signal is at some distance to the right of the left-hand side of the screen. Some employ a fixed time delay so that the sweep starts a predetermined time after the ignition signal. The disadvantage in this approach is that variations in engine speed will cause the ignition signal to appear at different places on the screen.
There have been several efforts to design a pretrigger circuit for an oscilloscope that takes into account any changes in the engine speed so that the display on the screen starts at a time irrespective of speed. For example, reference is made to U.S. Pat. No. 2,928,040 to Welcome, U.S. Pat. No. 2,985,011 to Sackett, U.S. Pat. Nos. 3,638,107 and 3,714,499 to Pelta et al. and U.S. Pat. No. 4,008,434 to Schaefer. The circuits disclosed in these patents do not insure constant location of the wave form on the face of the oscilloscope, with changes in engine speed and dwell.